France's justice minister has defied calls for her to quit after it emerged that she knew former President Nicolas Sarkozy's phone was being tapped, apparently contradicting an earlier statement which she made.

Mr Sarkozy's opposition party has accused the government of using the surveillance, ordered as part of a party funding inquiry, to discredit them before this month's local elections in which President Francois Hollande's Socialists risk losing ground.

Justice minister Christiane Taubira dismissed that accusation on Monday, saying she had not been informed about the phone-tapping before Le Monde newspaper revealed it last week.

Barely 24 hours later, Prime Minister Jean-Marc Ayrault acknowledged on television that he and Ms Taubira did know of the surveillance. The opposition then called for Ms Taubira to resign.

But the justice minister, in a hastily arranged appearance at a regular news briefing after the weekly cabinet meeting, rejected those demands, saying she had been misunderstood.

"I did not have, and do not have, information on the date, length and content of the surveillance," she said, adding that she "could have been more precise" in her initial statement.

"No, I did not lie," she said. "No, I will not resign."

Further complicating Ms Taubira's position, Le Monde said it had photographed official documents she had waved in the air during the news briefing.

It published excerpts from one of the documents, a letter sent by the prosecutor to the justice ministry, that appeared to contradict her statements that she was not aware of the dates of the phone-tapping and how long it lasted.

Mr Ayrault gave his support to Ms Taubira as he emerged from the cabinet meeting minutes later, telling reporters: "She has her place in the government."

Despite Mr Ayrault's comments supporting Ms Taubira, the affair has put Mr Hollande's government on the defensive.

The opposition UMP party had itself been on the backfoot over accusations of irregular party funding by its leader Jean-Francois Cope, which he denies, and leaked audio tapes revealing tensions in its leadership.

Investigators launched the phone-tapping last year after allegations that the late Libyan leader Muammar Gaddafi had funded Mr Sarkozy's 2007 election campaign, a legal case that could yet cloud any political come-back by the 59-year-old.

Mr Sarkozy, who has hinted he may run for president in 2017 after Mr Hollande ousted him in 2012, has denied all wrongdoing.    

French voters go to the polls on March 23 and 30 to elect new city mayors in the first major mid-term test of Mr Hollande's popularity since he took office in May 2012.

His poll ratings are at record lows for failing to reduce unemployment and start turning around the eurozone's second-largest economy.

Christian Jacob, parliamentary speaker for the conservative MP party, said the government's admission that it was aware of the phone-tapping was "extremely serious" and demanded an emergency session of parliament on the matter.